Methods and structures for expanding a memory operation window and reducing a second bit effect

ABSTRACT

Methods and structures are described for increasing a memory operation window in a charge trapping memory having a plurality of memory cells in which each memory cell is capable of storing multiple bits per memory cell. In a first aspect of the invention, a first method to increase a memory operation window in a two-bit-per-cell memory is described by applying a positive gate voltage, +Vg, to erase a memory cell to a negative voltage level. Alternatively, a negative gate voltage, −Vg, is applied to the two-bit-per-cell memory for erasing the memory cell to a negative voltage level. A second method to increase a memory operation window is to erase a memory cell to a voltage level that is lower than an initial voltage threshold level. These two erasing methods can be implemented either before a programming step (i.e., a pre-program erase operation) or after a programming step (i.e., a post-program erase operation).

CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to a concurrently filed and co-pending U.S.patent application Ser. No. 11/425,523, entitled “Memory Structures forExpanding a Second Bit Operation Window” by Chao-I Wu, owned by theassignee of this application and incorporated herein by reference.

This application relates to a concurrently filed previously, nowabandoned, U.S. patent application Ser. No. 11,425,541, entitled “TopDielectric Structures in Memory Devices and Methods for Expanding aSecond Bit Operation Window” by Chao-I Wu, owned by the assignee of thisapplication and incorporated herein by reference.

This application relates to a concurrently filed and co-pending U.S.patent application Ser. No. 11/425,553, entitled “Bottom DielectricStructures and High-K Memory Structures in Memory Devices and Methodsfor Expanding a Second Bit Operation Window” by Chao-I Wu, owned by theassignee of this application and incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electrically programmable anderasable memory, and more particularly, to methods and devices forincreasing a memory operation window and reducing a second bit effect inmulti-bit-per-cell operations.

2. Description of Related Art

Electrically programmable and erasable nonvolatile memory technologiesbased on charge storage structures known as Electrically ErasableProgrammable Read-Only Memory (EEPROM) and flash memory are used in avariety of modern applications. A flash memory is designed with an arrayof memory cells that can be independently programmed and read. Senseamplifiers in a flash memory are used to determine the data value orvalues stored in a nonvolatile memory. In a typical sensing scheme, anelectrical current through the memory cell being sensed is compared to areference current by a current sense amplifier.

A number of memory cell structures are used for EEPROM and flash memory.As the dimensions of integrated circuits shrink, greater interest isarising for memory cell structures based on charge trapping dielectriclayers, because of the scalability and simplicity of the manufacturingprocesses. Memory cell structures based on charge trapping dielectriclayers include structures known by the industry names Nitride Read-OnlyMemory (NROM), SONOS, and PHINES, for example. These memory cellstructures store data by trapping charge in a charge trapping dielectriclayer, such as silicon nitride. As negative charge is trapped, thethreshold voltage of the memory cell increases. The threshold voltage ofthe memory cell is reduced by removing negative charge from the chargetrapping layer.

NROM devices use a relatively thick bottom oxide, e.g. greater than 3nanometers, and typically about 5 to 9 nanometers, to prevent chargeloss. Instead of direct tunneling, band-to-band tunneling induced hothole injection BTBTHH can be used to erase the cell. However, the hothole injection causes oxide damage, leading to charge loss in the highthreshold cell and charge gain in the low threshold cell. Moreover, theerase time must be increased gradually during program and erase cyclingdue to the hard-to-erase accumulation of charge in the charge trappingstructure. This accumulation of charge occurs because the hole injectionpoint and electron injection point do not coincide with each other, andsome electrons remain after the erase pulse. In addition, during thesector erase of an NROM flash memory device, the erase speed for eachcell is different because of process variations (such as channel lengthvariation). This difference in erase speed results in a large Vtdistribution of the erase state, where some of the cells become hard toerase and some of them are over-erased. Thus the target threshold Vtwindow is closed after many program and erase cycles and poor enduranceis observed. This phenomenon will become more serious when thetechnology keeps scaling down.

A traditional floating gate device stores 1 bit of charge in aconductive floating gate. The advent of NROM cells in which each NROMcell provides 2 bits of flash cells that store charge in anOxide-Nitride-Oxide (ONO) dielectric. In a typical structure of an NROMmemory cell, a nitride layer is used as a trapping material positionedbetween a top oxide layer and a bottom oxide layer. The charge in theONO dielectric with a nitride layer may be either trapped on the leftside, i.e. the left bit, or the right side, i.e. the right bit, of anNROM cell. An operation applied to the left bit affects the right bit,or vice versa, which is known as a second bit effect. The second biteffect impacts an operation window of the NROM cell.

A frequently used technique to program NROM cells in an NROM array isthe hot electron injection method. During an erase operation, a commontechnique used to erase memory cells is called the band-to-bandtunneling hot hole injection. The intrinsic issue of second bit effectaffects the operation window. The second bit effect is caused by theinteraction of a left bit and a right bit in the NROM memory cell. It isdesirable to have methods and devices that increase a memory operationwindow in a charge trapping memory so that the second bit effect issignificantly reduced.

SUMMARY OF THE INVENTION

The present invention describes methods for increasing a memoryoperation window in a charge trapping memory having a plurality ofmemory cells in which each memory cell is capable of storing multiplebits per memory cell. In a first aspect of the invention, a first methodto increase a memory operation window in a two-bit-per-cell memory isdescribed by applying a positive gate voltage, +Vg, to erase a memorycell to a negative voltage level. Alternatively, a negative gatevoltage, −Vg, is applied to the two-bit-per-cell memory for erasing thecharge trapping memory to a negative voltage level. A second method toincrease a memory operation window is achieved by erasing the chargetrapping memory to a voltage level that is lower than an initial voltagethreshold level, Vt(i). These two methods of erasing a charge trappingmemory to either a negative voltage level or to a voltage level that isless than the initial voltage threshold level are also referred to asturn-on mode (TOM) methods. The two erase methods can be implementedeither before a programming step (i.e., a pre-program erase operation),or after a programming step (i.e., a post-program erase operation).

Two exemplary erase operations are illustrated in the following threeembodiments for implementing the present invention. The two eraseoperations include a hole injection erase operation and a band-to-bandhot hole erase operation. In a first embodiment, the charge trappingmemory is erased using a hole injection by a hole tunneling erase with apositive voltage. In a second embodiment, the charge trapping memory iserased using a hole injection by a hole tunneling erase with a negativevoltage. In a third embodiment, the charge trapping memory is erasedusing a band-to-band hot hole operation. A programming technique that issuitable for operation with these erase operations of a charge trappingmemory includes a channel hot electron (CHE).

The methods of the present invention are applied to a wide variety ofmemory devices that have a charge trapping structure, including but notlimited to memory devices having a nitride-oxide structure, anoxide-nitride-oxide structure, an nitride-oxide-nitride-oxide structureand an oxide-nitride-oxide-nitride-oxide structure. For example, in anMNOS memory device, a charge trapping layer overlies a dielectric layerwithout the presence of a dielectric layer that is disposed over thecharge trapping layer. Instead, a poly layer is formed over the chargetrapping layer. The nitride-oxide structure without a dielectric layerenables holes to be injected readily from the poly layer to the chargetrapping layer.

In a second aspect of the invention, a memory device in an MNOS-SOIstructure is described to increase a memory operation window whilereducing a second bit effect. A channel is formed between a sourceregion and a drain region without the need to apply a gate bias voltage,Vg. The MNOS-SOI memory comprises a charge trapping structure overlyingthe channel, where the charge trapping structure includes siliconnitride disposed over a dielectric layer. Alternatively, the memorydevice is implemented in a MONOS-SOI memory comprising a charge trappingstructure having an oxide-nitride-oxide stack. A suitable material tomanufacture the channel includes an epitaxy silicon or a poly silicon.The erase operation of a hole tunneling erase or a band-to-band hot holeerase can be applied in combination with the channel hot electrontechniques.

In a third aspect of the invention, a memory device in an MNONOSstructure is described with the application of a turn-on mode method toincrease an operation window while reducing a second bit effect. TheMNONOS memory structure comprises a top oxide structure having a siliconnitride layer overlying a dielectric layer. Alternatively, the memorydevice is implemented in a MONONOS structure that has a top oxidestructure of an oxide-nitride-oxide stack. The memory device with a topoxide structure can also be implemented on a thin-film transistor (TFT)structure by fabricating the memory device on a poly substrate, ratherthan a silicon substrate. Therefore, other embodiments of the memorydevice include MNONOS TFT memory structure and MONONOS TFT memorystructure. The erase operation of a hole tunneling erase or aband-to-band hot hole erase can be applied in combination with a channelhot electron technique. The turn-on mode operation can utilize both ahigh voltage memory operation and a low voltage memory operation. In thelow voltage memory operation, a voltage of less than about plus or minus+/−8 volts can be selected to carry out the erase operation.

In a fourth aspect of the invention, a charge trapping memory in aMONONS structure is described with the application of the turn-on modemethod to increase an operation window and reducing a second bit effect.The MONONS memory structure comprises a bottom oxide structure having adielectric layer overlying a silicon nitride layer. Alternatively, thememory device is implemented in a MONONOS structure comprising a bottomoxide structure having an oxide-nitride-oxide stack. The memory devicewith a bottom oxide structure can also be implemented on a thin-filmtransistor (TFT) structure by fabricating the memory device on a polysubstrate, rather than a silicon substrate. Therefore, other embodimentsof the memory device include MONONS TFT memory structure and MONONOS TFTmemory structure. In a further embodiment, the charge trapping memorycomprising a high-K material overlying a charge trapping layer on asilicon substrate, M(HK)NOS structure, or on a poly substrate, M(HK)NOSTFT structure. The erase operation of a hole tunneling erase or aband-to-band hot hole erase can be applied in combination with thechannel hot electron technique. The turn-on mode operation can utilizeboth a high voltage memory operation and a low voltage memory operation.In the low voltage memory operation, a voltage of less than about plusor minus +/−8 volts can be selected to carry out the erase operation.

Advantageously, the present invention provides methods and structuresfor increasing a memory operation window in a charge trapping memory andreducing the second bit effect. The present invention is also applicableto low voltage memory applications.

The structures and methods of the present invention are disclosed in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims. These and otherembodiments, features, aspects, and advantages of the invention willbecome better understood with reference to the following description,claims and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to specific embodimentsthereof, and reference will be made to the drawings, in which:

FIG. 1A illustrates a simplified structural diagram of an exemplarycharge trapping memory cell in an MNOS structure in accordance with thepresent invention;

FIG. 1B is a structural diagram illustrating the programming of thecharge trapping memory cell by channel hot electron programming of aright bit in accordance with the present invention; FIG. 1C is astructural diagram illustrating the programming of the charge trappingmemory cell by channel hot electron programming of a left bit inaccordance with the present invention; and FIG. 1D is a structuraldiagram illustrating a hole injection erase at a channel region of thecharge trapping memory in accordance with the present invention.

FIG. 2 is a structural diagram illustrating a first embodiment of anerase method by employing a hole tunneling erase with a positive gatevoltage to a negative voltage threshold from a gate terminal in a SONOSmemory in accordance with the present invention.

FIG. 3 is a structural diagram illustrating a second embodiment of theerase method by employing a hole tunneling erase with a negative gatevoltage to a negative voltage threshold from a substrate in a SONOSmemory in accordance with the present invention.

FIGS. 4A-4B are structural diagrams illustrating a third embodiment ofthe erase method by employing a band-to-band hot hole erase of the SONOSmemory to a negative voltage threshold in accordance with the presentinvention.

FIG. 5 is a flow diagram illustrating the process in the firstembodiment of the erase method by hole tunneling with a positive gatevoltage in accordance with the present invention.

FIG. 6 is a flow diagram illustrating the process in the secondembodiment of the erase method by hole tunneling with a negative gatevoltage in accordance with the present invention.

FIG. 7 is a flow diagram illustrating the process in the thirdembodiment of the erase method by band-to-band hot hole erase inaccordance with the present invention.

FIG. 8A is a structural diagram illustrating the programming of the leftbit in a MNOS structure in accordance with the present invention; andFIG. 8B is a corresponding graphical diagram illustrating the second biteffect, which in this instance refers to the right bit in accordancewith the present invention.

FIGS. 9A-B are graphical diagrams illustrating a second bit window of anMNOS memory cell with a voltage threshold of about zero volts with anotation of Vt in FIG. 9A and with a notation of Vt shift in FIG. 9B inaccordance with the present invention.

FIG. 10A and FIG. 10B is a graphical diagram illustrating a second bitwindow of an MNOS memory cell with a voltage threshold of negativevoltage threshold level with a notation of Vt in FIG. 10A and with anotation of Vt shift in FIG. 10B in accordance with the presentinvention.

FIG. 11 is a process diagram illustrating a first embodiment implementedin a MNOS-SOI memory in accordance with the present invention.

FIG. 12 is a process diagram illustrating a second embodimentimplemented in a MONOS-SOI memory in accordance with the presentinvention.

FIGS. 13A-13C are structural diagrams illustrating a first embodiment ofan erase operation by hole tunneling erase in the MNOS-SOI memory inaccordance with the present invention.

FIGS. 14A-14D are structural diagrams illustrating a second embodimentof an erase operation by band-to-band hot hole erase in the MNOS-SOImemory in accordance with the present invention.

FIG. 15A is a structural diagram illustrating the programming of theleft bit in the MNOS-SOI structure in accordance with the presentinvention; and FIG. 15B is a corresponding graphical diagramillustrating the second bit effect of the right bit in accordance withthe present invention.

FIG. 16 illustrates a first embodiment of a top oxide with a multi-layerdielectric structure implemented in an MNONOS thin film transistormemory for use with a turn-on mode operation in accordance with thepresent invention.

FIG. 17 illustrates a second embodiment of a top oxide with amulti-layer stack structure implemented in an MONONOS memory for use inthe turn-on mode operation in accordance with the present invention.

FIGS. 18A-18C are structural diagrams illustrating a first method forincreasing a second bit window in a top multi-layer dielectric structurefor use in the turn-on mode operation, which are applicable to both thefirst and second embodiments of the MNONOS memory and the MNONONOSmemory, in accordance with the present invention.

FIGS. 19A-19C are structural diagrams illustrating a second method forincreasing a second bit window in the top multi-layer dielectricstructure for use in the turn-on mode operation, which are applicable toboth the first and second embodiments of the MNONOS memory and theMNONONOS memory, in accordance with the present invention.

FIG. 20A is a structural diagram illustrating the programming of theleft bit in the MNONOS memory or the MNONONOS memory in accordance withthe present invention; and FIG. 20B is a corresponding graphical diagramillustrating the second bit effect of the right bit in accordance withthe present invention.

FIG. 21 illustrates a first embodiment of a bottom oxide with amulti-layer dielectric structure implemented in a MONONS memory for usein a turn-on mode operation in accordance with the present invention.

FIG. 22 illustrates a second embodiment of the bottom oxide with themulti-layer dielectric structure implemented in a MONONOS memory for usein the turn-on mode operation in accordance with the present invention.

FIG. 23 illustrates a third embodiment of the bottom oxide with themulti-layer dielectric structure implemented in a MONONS TFT memory on apoly substrate for use in the turn-on mode operation in accordance withthe present invention.

FIG. 24 illustrates a fourth embodiment of the bottom oxide with themulti-layer dielectric structure implemented in a MONONOS TFT memory ona poly substrate for use in the turn-on mode operation in accordancewith the present invention.

FIG. 25 illustrates a first embodiment of a M(HK)NOS memory structurehaving two bits per cell with a high-K material stack on a siliconsubstrate for use in the turn-on mode operation in accordance with thepresent invention.

FIG. 26 illustrates a second embodiment of a M(HK)NOS memory structurewith a high-K material stack on a poly substrate for use in the turn-onmode operation in accordance with the present invention.

FIGS. 27A-27C are structural diagrams illustrating a first method forincreasing a second bit window of a M(HK)NOS memory structure with ahigh-K material stack on either a silicon substrate or a poly substratefor use in the turn-on mode operation in accordance with the presentinvention.

FIGS. 28A-28C are structural diagrams illustrating a second method forincreasing a second bit window of a M(HK)NOS memory structure with ahigh-K material stack on either a silicon substrate or a poly substratefor use in a turn-on mode operation in accordance with the presentinvention.

FIG. 29A is a structural diagram illustrating the programming of theleft bit in the M(HK)NOS memory or the M(HK)NOS TFT memory in accordancewith the present invention; and FIG. 29B is a corresponding graphicaldiagram illustrating the second bit effect of the right bit inaccordance with the present invention.

FIG. 30 is a flow diagram illustrating the process to pre-program eraseSONOS-type or TFT-SONOS memories by applying a positive gate voltage inaccordance with the present invention.

FIG. 31 is a flow diagram illustrating the process to pre-program eraseSONOS-type or TFT-SONOS memories by applying a negative gate voltage inaccordance with the present invention.

FIG. 32 is a flow diagram illustrating the process to pre-program erasea SONOS-type or TFT-SONOS memory having a top oxide structure inaccordance with the present invention.

FIG. 33 is a flow diagram illustrating the process to pre-program erasea SONGS-type or TFT-SONOS memory having a bottom oxide structure inaccordance with the present invention.

FIG. 34 is a flow diagram illustrating the process to pre-program erasea SONOS-type or TFT-SONOS memory comprising a high-K material inaccordance with the present invention.

DETAILED DESCRIPTION

A description of structural embodiments and methods of the presentinvention is provided with reference to FIGS. 1-34. It is to beunderstood that there is no intention to limit the invention to thespecifically disclosed embodiments but that the invention may bepracticed using other features, elements, methods and embodiments. Likeelements in various embodiments are commonly referred to with likereference numerals.

In a first aspect of the invention, referring now to FIG. 1A, there isshown a simplified structural diagram illustrating an exemplary chargetrapping memory cell 100 in an MNOS structure. The charge trappingmemory cell 100 has a p-type substrate 110 with n+ doped regions 112 and114. A bottom dielectric structure 120 (bottom oxide) overlays thesubstrate 110, a charge trapping structure 130 (e.g., a silicon nitridelayer) overlays the bottom dielectric structure 120, and a p-poly 140overlays the charge trapping structure 130. A gate voltage 150, Vg, isapplied to the p-poly 140, and a substrate voltage 152, Vsub, is appliedto the p-substrate 110. A drain voltage Vd 158 is applied to the n+doped region 114, and a source voltage Vs 158 is applied to the n+ dopedregion 112.

The MNOS structure in the charge trapping memory cell 100 is intended asan illustration for implementing the present method invention. The MNOSstructure has a nitride-oxide stack without a top oxide, whichadvantageously allows holes to enter directly into the charge trappingstructure 130 without the presence of a top oxide. Other combinations ofcharge trapping structures, such as oxide-nitride-oxide (ONO), oroxide-nitride-oxide-nitride-oxide (ONONO) stack can be implementedwithout departing from the spirit of the present invention. The p-poly140 can be implemented with a wide variety of materials including polyor metal.

FIG. 1B illustrates a structural diagram of the programming of thecharge trapping memory cell 100 by channel hot electron at a right bit162. The phrases, “left bit” and “right bit”, are used herein to referto the “left” and “right” charge storage sites on opposite sides of adielectric charge storage structure. A directional arrow 160 indicatesthat the channel hot electron is applied to the right bit 162, as shownwith electrons in the charge trapping structure 130. The gate voltage Vg150 of 8 volts is applied, the drain voltage Vd 156 of 5 volts isapplied, the source voltage Vs 158 of 0 volts is applied, and thesubstrate voltage Vsub 152 of 0 volts is applied. The combination ofthese applied voltages results in channel hot electron of the right bitin the charge trapping memory 100 to a high positive voltage threshold+Vt.

The bias condition for the drain and source regions 112, 114 is switchedto carry out the programming of the other bit in the charge trappingmemory 100. FIG. 1C is a structural diagram illustrating the programmingof the charge trapping memory 100 by channel hot electron of a left bit.A directional arrow 170 indicates that the channel hot electron isapplied to a left bit, as shown with electrons 172 in the chargetrapping structure 130. The gate voltage Vg 150 of 8 volts is applied,the drain voltage Vd 156 of 0 volts is applied, the source voltage Vs158 of 5 volts is applied, and the substrate voltage Vsub 152 of 0 voltsis applied. The combination of these applied voltages results in channelhot electron of the left bit of the charge trapping memory cell 100 to ahigh positive voltage threshold +Vt.

FIG. 1D is a structural diagram illustrating a hole injection (HI) eraseat a channel region of the charge trapping memory cell 100. The term“hole injection” is also referred to as “hole tunneling.” A holeinjection erase is typically not a conventional erase method. Whenapplying a positive gate voltage in hole injection, holes 180 can beinjected from the gate to the charge trapping structure 130. The gatevoltage Vg 150 of 16 volts is applied, the drain voltage Vd 156 of 0volts is applied, the source voltage Vs 158 of 0 volts is applied, andthe substrate voltage Vsub 152 of 0 volts is applied. The combination ofthese applied voltages results in the left bit and the right bit of thecharge trapping memory cell 100 to a negative voltage threshold −Vt.

As generally used herein, programming refers to raising the thresholdvoltage of a memory cell and erasing refers to lowering the thresholdvoltage of a memory cell. However, the invention encompasses bothproducts and methods where programming refers to raising the thresholdvoltage of a memory cell and erasing refers to lowering the thresholdvoltage of a memory cell, and products and methods where programmingrefers to lowering the threshold voltage of a memory cell and eraserefers to raising the threshold voltage of a memory cell.

Representative top dielectrics include silicon dioxide and siliconoxynitride having a thickness of about 5 to 10 nanometers, or othersimilar high dielectric constant materials including for example Al₂O₃.Representative bottom dielectrics include silicon dioxide and siliconoxynitride having a thickness of about 3 to 10 nanometers, or othersimilar high dielectric constant materials. Representative chargetrapping structures include silicon nitride having a thickness of about3 to 9 nanometers, or other similar high dielectric constant materials,including metal oxides such as Al₂O₃, HfO₂, CeO₂, and others. The chargetrapping structure may be a discontinuous set of pockets or particles ofcharge trapping material, or a continuous layer as shown in the drawing.The charge trapping structure 130 has trapped charge such as representedby electrons.

Turning now to FIG. 2, there is shown a structural diagram illustratinga first embodiment of an erase method by employing a hole tunnelingerase of the SONOS memory 200 to a negative voltage threshold byapplying a positive gate voltage from a gate terminal of the SONOSmemory 200. The SONOS memory 200 comprises a charge trapping structure212 overlaying a first dielectric layer 210, and a second dielectriclayer 214 overlaying the charge trapping structure 212. An n-poly layer220 overlies the second dielectric layer 214. A high bias voltageapplied at a gate terminal causes a band distortion so that the seconddielectric layer 214 may be thinner at certain regions to allow holes topenetrate through the second dielectric layer 214. When a high biasvoltage is applied to a gate terminal in the n-poly 220, holes areinjected from the gate terminal (as indicated by arrows 240 a, 240 b),through the second dielectric layer 214, and to the charge trappingstructure 212. The second dielectric layer 214 may be selected to besufficiently thin for hole tunneling through the second dielectric layer214. A gate voltage Vg 230 is applied with a positive voltage of 16volts, a drain voltage Vd 234 is applied with 0 volts, a source voltageVs 236 is applied with 0 volts, and a substrate voltage Vsub 232 isapplied with 0 volts, The combination of these applied voltages resultsin hole tunneling erase of the SONOS memory 200 to the negative voltagethreshold −Vt, thereby increasing a memory operational window andreducing the second bit effect.

In FIG. 3, there is shown a structural diagram illustrating a secondembodiment of the erase method by applying a hole tunneling erase to aSONOS memory cell 300 to bring the memory cell to a negative voltagethreshold by applying a negative gate voltage from a substrate of aSONOS memory cell 300. The SONOS memory cell 300 comprises a chargetrapping structure 312 overlaying a first dielectric layer 310, and asecond dielectric layer 314 overlaying the charge trapping structure312. An n-poly layer 320 overlies the second dielectric layer 314. Ahigh negative bias voltage applied at a substrate 302 causes a banddistortion so that the first dielectric layer 310 may be thinner atcertain regions to allow holes to penetrate through first dielectriclayer 310. When a high negative bias voltage is applied to the substrate302, holes are injected from the substrate 302 (as indicated by arrows340 a, 340 b), through the first dielectric layer 310, and to the chargetrapping structure 312. The first dielectric layer 310 may be selectedto be sufficiently thin for hole tunneling through the first dielectriclayer 310. A gate voltage Vg 330 is applied with a negative voltage of−16 volts, a drain voltage Vd 334 is applied with 0 volts, a sourcevoltage Vs 336 is applied with 0 volts, and a substrate voltage Vsub 332is applied with 0 volts. The combination of these applied voltagesresults in hole tunneling erase of the SONOS memory 200 to a negativevoltage threshold −Vt, thereby increasing a memory operational windowand reducing the second bit effect.

FIGS. 4A-4B are structural diagrams illustrating a third embodiment ofthe erase method by employing a band-to-band hot hole erase to anegative voltage threshold in the SONOS memory cell 300. The eraseoperation of a right bit in the SONOS memory cell 300 is illustrated inFIG. 4A and the erase operation of a left bit in the SONOS memory cell300 is illustrated in FIG. 4B. When erasing a right bit using aband-to-band hot hole erase, a drain voltage Vd 434 is applied with 5volts and a source voltage Vs 436 is applied with 0 volts in order tomove holes toward the right side of a charge trapping structure 410, asindicated by an arrow 420. The bias voltage conditions are reversed inerasing a left bit. When erasing a left bit using a band-to-band hothole erase, the source voltage Vs 436 is applied with 5 volts and thedrain voltage Vd 434 is applied with 0 volts, as indicated by an arrow422. In both erase operations of the right bit and the left bit, a gatevoltage Vg 430 is applied with 8 volts and a substrate voltage Vs 432 isapplied with 0 volts.

Alternatively, the erase methods in the first, second and thirdembodiments are carried out to erase the SONOS memory to a voltage levelthat is lower than an initial voltage threshold, Vt(i), rather than to anegative voltage threshold Vt. Although the SONOS memory cell isillustrated above with respect to the first, second and thirdembodiments, other types of charge trapping memories are also applicableto the present invention, including other SONOS-type or TFT-SONOSmemories.

As shown in FIG. 5, there is a flow diagram illustrating the process 500in the first embodiment of the erase method by hole tunneling with apositive gate voltage. At a step 510, the SONOS memory cell 300 isprogrammed by using a channel hot electron technique. At step 520, theSONOS memory cell 300 is erased to a negative voltage threshold byapplying a positive gate voltage which causes hole tunneling erase fromthe gate terminal. The erase of the SONOS memory cell 300 to a negativevoltage threshold increases a memory operation window and reduces thesecond bit effect. Alternatively, the SONOS memory cell 300 is erased toa voltage level lower than an initial voltage threshold by applying apositive gate voltage from the gate terminal.

In FIG. 6, there is shown a flow diagram illustrating the process 600 inthe second embodiment of the erase method by hole tunneling with anegative gate voltage. At a step 610, the SONOS memory cell 300 isprogrammed by using a channel hot electron technique. At step 620, theSONOS memory cell 300 is erased to a negative voltage threshold byapplying a negative gate voltage which causes hole tunneling erase fromthe substrate. The erase of the SONOS memory cell 300 to a negativevoltage threshold increases a memory operation window while reducing thesecond bit effect. Alternatively, the SONOS memory cell 300 is erased toa voltage level lower than an initial voltage threshold by applying anegative gate voltage from the substrate of the SONOS memory cell 300.

FIG. 7 is a flow diagram illustrating the process 700 in the thirdembodiment of the erase method by band-to-band hot hole erase. At step710, the SONOS memory cell 300 is programmed by using a channel hotelectron technique. At step 720, the SONOS memory cell 300 is erased toa negative voltage threshold by using a band-to-band hot hole erase. Theerase operation of the SONOS memory cell 300 to a negative voltagethreshold increases a memory operation window and reduces the second biteffect. Alternatively, the SONOS memory cell 300 is erased to a voltagelevel lower than an initial voltage threshold by using the band-to-bandhot hole erase technique.

FIG. 8A is a structural diagram illustrating the programming of the leftbit (Bit-L) in a MNOS structure, and FIG. 8B is a correspondinggraphical diagram of a two-bit-per-cell operation window thatillustrates the second bit effect, which in this instance refers to theright bit (Bit-R). A second bit effect occurs in a charge trappingmemory that employs a two-bit-per-cell operation, i.e. a left bit andright bit. When one of the two bits is programmed, the voltage thresholdfor the other bit may also increase even though only one bit is beingprogrammed. The programming of a left bit is illustrated in FIG. 8A withan indication of charges 810 on a left side 812. Although only the leftbit 812 is programmed, the programming of the left bit 812 also causesthe voltage threshold of a right bit 814 to increase, as shown in FIG.8B. A curve 820 illustrates that the voltage threshold of the right bit814 drifts higher as the left bit 812 is being programmed. Suchphenomenon is referred to as a second bit effect. An ideal curve,without the second bit effect, would show that a continuing programmingof a left bit would cause the voltage threshold of the left bit toincrease but the voltage threshold of the right bit would not beaffected such that the voltage threshold of the right bit would remainsubstantially constant.

FIGS. 9A-B are graphical diagrams illustrating a second bit window of anMNOS memory cell with a voltage threshold of about zero volts with anotation of Vt in FIG. 9A and with a notation of Vt shift in FIG. 9B. Asecond bit window is defined as the difference between the shift in thevoltage threshold of the right bit Vt(r) and the shift in the voltagethreshold of the left bit Vt(l). As depicted in FIG. 9B, the voltagethreshold of the left bit has shifted to about 3.5 volts, and thevoltage threshold of the right bit has shifted to about 1.1 volts.Therefore, the second bit window in this instance is calculated as thedifference between the shift in Vt(l) and shift in Vt(r), which iscomputed as follows: 3.5 volts−1.1 volts=2.4 volts.

FIG. 10A and FIG. 10B is a graphical diagram illustrating a second bitwindow of an MNOS memory cell with a negative voltage threshold levelwith a notation of Vt in FIG. 10A and with a notation of Vt shift inFIG. 10B. As depicted in FIG. 10B, the voltage threshold of the left bithas shifted to about 6.0 volts, and the voltage threshold of the rightbit has shifted to about 1.5 volts. Therefore, the second bit window inthis instance is calculated as the difference between the shift in Vt(l)and shift in Vt(r), which is computed as follows: 6.0 volts−1.5volts=4.5 volts. In comparison between erasing to about zero volts levelas shown in FIG. 9A and erasing to a negative voltage threshold level asshown in FIG. 10A, the second bit window is significantly larger for anerase operation to a negative voltage threshold level than an eraseoperation to about zero volts.

In a second aspect of the invention, FIG. 11 is a process diagramillustrating a first embodiment implemented in an MNOS-SOI (silicon oninsulator) memory 1100. The MNOS-SOI memory comprises an oxide layer1120 overlying a silicon substrate 1110 to serve as an insulatingmaterial. In a SOI structure, a channel 1130 is formed between an n+source region 1132 and an n+ drain region 1134 without applying a gatebias voltage Vg. The n+ source region 1132, the channel 1130 and the n+drain region 1134 overlie the oxide layer 1120. The channel 1130 isdeposited as a single crystal on the oxide 1120. The channel 1130 can beimplemented with epitaxy silicon or poly silicon. An example of asuitable thickness t 1190 of the channel 1130 ranges from about 500 Å toabout 1000 Å. A charge trapping layer 1150 overlies an oxide layer 1140,which is also referred to as a nitride-oxide (NO) stack. A poly gate1160 overlies the charge trapping layer 1150. Some suitable materialsfor implementing the poly gate 1160 include an n-poly, a p-poly, or ametal gate. Without the presence of a top oxide overlying the chargetrapping layer 1150, the erase operation, in using a hole tunnelinginjection, is able to more readily move holes through the poly gate andinto the charge trapping layer 1150. A gate bias voltage 1170 isconnected to the poly gate 1160, a source voltage 1172 is connected tothe n+ source region 1132, a drain voltage 1174 is connected to the n+drain region 1134, and a substrate voltage 1176 is connected to thesilicon substrate 1110.

FIG. 12 is a process diagram illustrating a second embodimentimplemented in a MONOS-SOT memory 1200. The MONOS-SOI memory comprisesan oxide layer 1120 overlying a silicon substrate 1210 to serve as aninsulating material. In a SOI structure, a channel 1230 is formedbetween an n+ source region 1232 and an n+ drain region 1234 withoutapplying a gate bias voltage Vg. The n+ source region 1232, the channel1230 and the n+ drain region 1234 overlie the oxide layer 1220. Thechannel 1230 is deposited as a single crystal on the oxide 1220. Thechannel 1230 can be implemented with epitaxy silicon or poly silicon. Anexample of a suitable thickness t 1290 of the channel 1230 ranges fromabout 500 Å to about 1000 Å. A charge trapping layer 1250 overlies abottom oxide layer 1240 and a top oxide layer 1260 overlies the chargetrapping layer 1250, which are also referred to as anoxide-nitride-oxide stack. A poly gate 1270 overlies the top oxide layer1260. Some suitable materials for implementing the poly gate 1270include an n-poly, a p-poly, or a metal gate. In one embodiment, the topoxide layer 1260 is selected to be sufficiently thin so that holes areable to move through the poly gate 1270 and the top oxide layer 1260 toreach the charge trapping layer 1250 by hole tunneling injection. A gatebias voltage 1280 is connected to the poly gate 1270, a source voltage1282 is connected to the n+ source region 1232, a drain voltage 1284 isconnected to the n+drain region 1234, and a substrate voltage 1286 isconnected to the silicon substrate 1210.

FIGS. 13A-13C are structural diagrams illustrating a first embodiment ofan erase operation by hole tunneling erase in the MNOS-SOI memory 1100or the MONOS-SOI memory 1200. In FIG. 13A, a channel hot electron isapplied on a right bit of the MNOS-SOI memory 1100, as indicated by anarrow 1310 moving in the direction toward the right, and an electron1320 is injected on the right side of the charge trapping layer 1150.The gate voltage Vg is applied with 10 volts, the substrate voltage Vsubis applied with 0 volts, the source voltage Vs is applied with zerovolts, and the drain voltage Vd is applied with 5 volts. The voltagebiasing in the source voltage Vs 1172 and the drain voltage Vd 1174 isreversed to conduct a channel hot electron on the left bit as shown inFIG. 13B by an arrow 1330 moving toward the left and an electron 1340 isinjected on the left side of the charge trapping layer 1150. The sourcevoltage Vs is applied with 5 volts, and the drain voltage is appliedwith 0 volts. During an erase operation, as shown in FIG. 13C, the gatevoltage Vg 1170 is applied with a positive voltage of +16 volts, thesubstrate voltage Vs 1176 is applied with 0 volts, the source voltage Vs1172 is applied with 0 volts, and the drain voltage Vd 1174 is appliedwith 0 volts. The hole tunneling erase operation causes holes 1350 topenetrate through the poly gate 1160 as indicated by arrows 1360 andinto the charge trapping layer 1150.

FIGS. 14A-14D are structural diagrams illustrating a second embodimentof an erase operation by band-to-band hot hole erase in the MNOS-SOImemory 11100 or the MONOS-SOI memory 1200. In FIG. 14A, a channel hotelectron is applied on a right bit, Bit-R, of the MNOS-SOI memory 1100,as indicated by an arrow 1410 moving in the direction toward the rightand an electron 1420 is injected on the right side of the chargetrapping layer 1150. The gate voltage Vg is applied with 10 volts, thesubstrate voltage Vsub is applied with 0 volts, the source voltage Vs isapplied with 0 volts, and the drain voltage Vd is applied with 5 volts.The voltage biasing in the source voltage Vs 1172 and the drain voltageVd 1174 is reversed to conduct a channel hot electron on the left bit,as shown in FIG. 14B by an arrow 1430 moving toward the left and anelectron 1440 injected on the left side of the charge trapping layer1140. The source voltage Vs is applied with 5 volts, and the drainvoltage is applied with 0 volts. An erase operation is carried out usinga band-to-band hot hole erase on a right bit as shown in FIG. 14C and ona left bit, as shown in FIG. 14D. The gate voltage Vg 1170 is appliedwith a positive voltage of +10 volts, the substrate voltage Vs 1176 isapplied with 0 volts, the source voltage Vs 1172 is applied with 0volts, and the drain voltage Vd 1174 is applied with 5 volts. Theband-to-band hot hole erase on the right bit causes holes 1450 to movefrom the n+ drain region 1134 into the channel 1130, through the oxidelayer 1140, and into the charge trapping layer 1150, as indicated by anarrow 1460. The gate voltage Vg 1170 is applied with a negative voltageof −10 volts, the substrate voltage Vs 1176 is applied with 5 volts, thesource voltage Vs 1172 is applied with 0 volts, and the drain voltage Vd1174 is applied with 0 volts. The band-to-band hot hole erase on theleft bit causes holes 1470 to move from the n+ source region 1132 intothe channel 1130, through the oxide layer 1140, and into the chargetrapping layer 1150, as indicated by an arrow 1480.

FIG. 15A is a structural diagram illustrating the programming of theleft bit (Bit-L) in the MNOS-SOI memory 1100 or the MONOS-SOI memory1200, and FIG. 15B is a corresponding graphical diagram of atwo-bit-per-cell operation window that illustrates the second biteffect, which in this instance refers to the right bit (Bit-R). A secondbit effect occurs in a memory cell that employs a two-bit operation,i.e. a left bit and right bit. When one of the two bits is programmed,the voltage threshold for the other bit may also increase even thoughonly one bit is programmed. The programming of a left bit is illustratedin FIG. 15A with an indication of charges 1510 on a left bit 1512.Although only the left bit 1512 is programmed, the programming of theleft bit 1512 also causes the voltage threshold of a right bit 1514 toincrease, as shown in FIG. 15B. A curve 1520 illustrates that thevoltage threshold of right bit 1514 increases as the left bit 1512 isprogrammed. Such a phenomenon is referred to as a second bit effect. Anideal curve, without the second bit effect, would reflect that acontinuing programming of a left bit would cause the voltage thresholdof the left bit to increase but the voltage threshold of the right bitwould not be affected such that the voltage threshold of the right bitremains substantially constant.

In a third aspect of the invention, FIG. 16 illustrates a firstembodiment of a top oxide with a multi-layer dielectric structureimplemented in an MNONOS memory 1600 comprising in a turn-on modeoperation. The MNONOS memory 1600 is fabricated on a silicon substrate1610. A drain n+ doped region 1620 and a source n+ doped region 1622 areformed on the upper right side and the upper left side of the p-typesilicon substrate 1610. A bottom dielectric structure 1630, such as anoxide, overlays the silicon substrate 1610 and a charge trapping layer1640 comprising a silicon nitride layer that overlays the bottomdielectric structure 1630. A top dielectric structure 1650 overlays thecharge trapping layer 1640. The top dielectric structure 1650 hasmultiple layers comprising a silicon nitride layer 1654 overlaying anoxide layer 1652, which is also referred to as an N—O stack. A p-polylayer 1660 overlays the top dielectric structure 1650. Other suitablematerials can be implemented in place of the p-poly layer 1660, such asn-poly or a metal gate. A gate voltage 1670, Vg, is applied to thep-poly 1660, and a substrate voltage 1672, Vsub, is applied to thep-type silicon substrate 1610. A drain voltage Vd 1674 is applied to thedrain n+ doped region 1620, and a source voltage Vs 1676 is applied tothe source n+ doped region 1622.

FIG. 17 illustrates a second embodiment of a top oxide with amulti-layer stack structure implemented in a MONONOS memory 1700 in aturn-on mode operation. The MNONONOS memory 1700 is fabricated on ap-type silicon substrate 1710, instead of a conventional siliconsubstrate. A drain n+ doped region 1720 and a source n+ doped region1722 are formed on the upper right side and the upper left side of thep-type silicon substrate 1710. A dielectric structure 1730, such as anoxide, overlays the substrate 1710 and a silicon nitride layer 1740overlays the bottom dielectric structure 1730. A top dielectricstructure 1750 overlays the silicon nitride 1740. The top dielectricstructure 1750 has multiple layers comprising an oxide 1756 overlaying asilicon nitride layer 1754, and the silicon nitride layer 1754overlaying an oxide layer 1752, which is also referred to as an O—N—Ostack. A p-poly layer 1760 overlays the top dielectric structure 1750.Other suitable materials can be implemented in place of the p-poly layer1760, such as n-poly or a metal gate. A gate voltage 1770, Vg, isapplied to the p-poly 1760, and a substrate voltage 1772, Vsub, isapplied to the p-type poly substrate 1710. A drain voltage Vd 1774 isapplied to the drain n+ doped region 1720, and a source voltage Vs 1776is applied to the source n+ doped region 1722.

FIGS. 18A-18C are structural diagrams illustrating a first method forincreasing a second bit window in a top multi-layer dielectric structurefor use in a turn-on mode operation, which are applicable to both thefirst and second embodiments of the MNONOS memory 1600 and the MNONONOSmemory 1700. FIG. 18A is a structural diagram illustrating theprogramming of the MNONOS memory 1600 by channel hot electron at a rightbit location. A directional arrow 1810 indicates that the channel hotelectron is applied to a right bit, as shown with electrons 1820 in thecharge trapping layer 1640. The gate voltage Vg 1670 is applied 8 volts,the drain voltage Vd 1674 is applied 5 volts, the source voltage Vs 1676is applied 0 volts, and the substrate voltage Vsub 1672 is applied 0volts. The combination of these applied voltages result in of the rightbit in the MNONOS memory 1600 to a positive voltage threshold +Vt.

FIG. 18B is a structural diagram illustrating the programming of theMNONOS memory 1600 by channel hot electron at a left bit location. Adirectional arrow 1830 indicates that the channel hot electron isapplied to the left bit, as shown with electrons 1840 in the chargetrapping layer 1640. The gate voltage Vg 1670 is applied 8 volts, thedrain voltage Vd 1674 is applied 0 volts, the source voltage Vs 1676 isapplied 5 volts, and the substrate voltage Vsub 1672 is applied 0 volts.The combination of these applied voltages result in channel hot electronof the left bit in the MNONOS memory 1600 to a positive voltagethreshold +Vt.

FIG. 18C is a structural diagram illustrating a hole injection erase ofthe MNONOS memory 1600 by hole tunneling. During the erase operation,the hole tunneling erase is carried out on a left bit in a direction asindicated by an arrow 1850 by moving hole charges 1860 a through thep-poly 1660, the silicon nitride layer 1654, and the oxide 1652 into thecharge trapping layer 1640. The hole tunneling erase is also carried outon a right bit by moving hole charges 1860 b through the p-poly 1660,the silicon nitride layer 1654, and the oxide 1652 into the chargetrapping layer 1640. The gate voltage Vg 1670 is applied with 16 volts,the drain voltage Vd 1674 is applied with 0 volts, the source voltage Vs1676 is applied with 0 volts, and the substrate voltage Vsub 1672 isapplied with 0 volts. The combination of these applied voltages causeshole injection erase by hole tunneling in moving hole charges throughthe p-poly 1660, the silicon nitride layer 1654, and the oxide 1652 intothe charge trapping layer 1640.

The gate bias voltage Vg can be modified so that it is suitable for alow voltage operation. FIGS. 19A-19C are structural diagramsillustrating a second method for increasing a second bit window in a topmulti-layer dielectric structure for use in a turn-on mode operation,which are applicable to both the first and second embodiments of theMNONOS memory 1600 and the MNONONOS memory 1700. FIGS. 19A-B arestructural diagrams illustrating the programming of the MNONOS memory1600 by channel hot electron at a right bit location and a left bitlocation, respectively, that are similar to the descriptions as in FIGS.18A-B. A directional arrow 1910 indicates that the channel hot electronis applied to a right bit location, as shown with electrons 1920 in thecharge trapping layer 1640. The gate voltage Vg 1670 is applied with 8volts, the drain voltage Vd 1674 is applied with 5 volts, the sourcevoltage Vs 1676 is applied with 0 volts, and the substrate voltage Vsub1672 is applied with 0 volts. The combination of these applied voltagesresults in channel hot electron of the right bit in the MNONOS memory1600 to a positive voltage threshold +Vt.

FIG. 19B is a structural diagram illustrating the programming of theMNONOS memory 1600 by channel hot electron at a left bit location. Adirectional arrow 1930 indicates that the channel hot electron isapplied to the left bit, as shown with electrons 1940 in the chargetrapping layer 1640. The gate voltage Vg 1670 is applied with 8 volts,the drain voltage Vd 1674 is applied with 0 volts, the source voltage Vs1676 is applied with 5 volts, and the substrate voltage Vsub 1672 isapplied with 0 volts. The combination of these applied voltages resultsin channel hot electron of the left bit in the MNONOS memory 1600 to apositive voltage threshold +Vt.

FIG. 19C is a structural diagram illustrating a hole injection erase ofthe MONOS memory 1600 by hole tunneling. During the erase operation, thehole tunneling erase is carried out on a left bit by moving hole charges1960 a through the p-poly 1660, the silicon nitride layer 1654, and theoxide 1652 into the charge trapping layer 1640. The hole tunneling eraseis applied to a right bit in a direction as indicated by an arrow 1950by moving hole charges 1960 b through the p-poly 1660, the siliconnitride layer 1654, and the oxide 1652 into the charge trapping layer1640. The gate voltage Vg 1670 is applied with 8 volts, the drainvoltage Vd 1674 is applied with 0 volts, the source voltage Vs 1676 isapplied with 0 volts, and the substrate voltage Vsub 1672 is appliedwith −8 volts. The combination of these applied voltages causes holeinjection erase by hole tunneling in moving hole charges through thep-poly 1660, the silicon nitride layer 1654, and the oxide 1652 into thecharge trapping layer 1640. The second operation method is suitable fora low voltage operation by reducing the gate bias voltage from +16 voltsto +8 volts, and by applying −8 volts to the silicon substrate 1610.

FIG. 20A is a structural diagram illustrating the programming of theleft bit in the MNONOS memory 1600 or the MNONONOS memory 1700, and FIG.20B is a corresponding graphical diagram of a two-bit-per-cell operationwindow that illustrates the second bit effect, which in this instancerefers to the right bit. A second bit effect occurs in a memory cellthat employs a two-bit operation, i.e. a left bit and right bit. Whenone of the two bits is programmed, the voltage threshold for the otherbit may also increase even though only one bit is programmed. Theprogramming of a left bit is illustrated in FIG. 20A with an indicationof charges 2010 on a left bit 2012. Although only the left bit 2012 isprogrammed, the programming of the left bit 2012 also causes the voltagethreshold of a right bit 2014 to increase, as shown in FIG. 20B. A curve2020 illustrates that the voltage threshold of right bit 2014 increasesas the left bit 2012 is programmed. Such phenomenon is referred to as asecond bit effect. An ideal curve, without the second bit effect, wouldinvolve a continuing programming of a left bit which would cause thevoltage threshold of the left bit to increase but the voltage thresholdof the right bit would not be affected such that the voltage thresholdof the right bit would remain substantially constant.

The MNONOS memory 1600 with the p-type silicon substrate and MONONOSmemory 1700 with the p-type silicon substrate are intended asillustrations for carrying out the turn-on mode operation in the thirdaspect of the invention with reference to FIGS. 16-20. Other memorystructures can also be practiced within the spirits of the presentinvention, including MNONOS TFT memory and MONONOS TFT memory.

In a fourth aspect of the invention, FIG. 21 illustrates a firstembodiment of a bottom oxide with a multi-layer dielectric structureimplemented in a MONONS memory 2100 for use in a turn-on mode operation.The MONONS memory 2100 is fabricated on a p-type silicon substrate 2110with a drain n+ doped region 2120 and a source n+ doped region 2122 thatare formed on the upper right side and the upper left side of the p-typesilicon substrate 2110, respectively. A bottom dielectric structure 2130overlays the p-type silicon substrate 2110. The bottom dielectricstructure 2130 has multiple layers comprising an oxide 2134 overlaying asilicon nitride layer 2132, which is also referred to as O—N layers. Asilicon nitride layer 2140 overlays the bottom dielectric structure2130, an oxide layer 2150 overlays the silicon nitride 2140, and ap-poly 2160 overlays the oxide layer 2150. Other suitable materials canbe implemented in place of the p-poly layer 2160, such as n-poly or ametal gate. A gate voltage 2170 Vg is applied to the p-poly 2160, and asubstrate voltage 2176 Vsub is applied to the p-type silicon substrate2110. A drain voltage Vd 2172 is applied to the drain n+ doped region2120, and a source voltage Vs 2174 is applied to the source n+ dopedregion 2122.

Referring now to FIG. 22, there is shown a third embodiment of a bottomoxide with a multi-layer dielectric structure implemented in a MONONOSmemory 2200 for use in a turn-on mode operation. The MONONOS memory 2200is fabricated on a p-type silicon substrate 2210 with a drain n+ dopedregion 2220 and a source n+ doped region 2222 that are formed on theupper right side and the upper left side of the p-type silicon substrate2210. A bottom dielectric structure 2230 overlays the p-type siliconsubstrate 2210. The bottom dielectric structure 2230 has multiple layerscomprising an oxide 2236 overlaying a silicon nitride layer 2234, andthe silicon nitride layer 2234 overlaying the oxide 2232, which is alsoreferred to as O—N—O layers. A silicon nitride layer 2240 overlays thebottom dielectric structure 2230, an oxide layer 2250 overlays thesilicon nitride 2240, and a p-poly 2260 overlays the oxide layer 2250.Other suitable materials can be implemented in place of the p-poly layer2260, such as n-poly or a metal gate. A gate voltage 2270 Vg, is appliedto the p-poly 2260, and a substrate voltage 2276 Vsub is applied to thep-type silicon substrate 2210. A drain voltage Vd 2272 is applied to thedrain n+ doped region 2220, and a source voltage Vs 2274 is applied tothe source n+ doped region 2222.

In FIG. 23, there is shown a third embodiment of a bottom oxide with amulti-layer dielectric structure implemented in a MONONS TFT memory 2300on a poly substrate for use in a turn-on mode operation. The MONONS TFTmemory 2300 is fabricated on a p-type poly substrate 2310 with a drainn+ doped region 2320 and a source n+ doped region 2322 that are formedon the upper right side and the upper left side of the p-type polysubstrate 2310, respectively. A bottom dielectric structure 2330overlays the p-type poly substrate 2310. The bottom dielectric structure2330 has multiple layers that comprise an oxide 2334 overlaying asilicon nitride layer 2332, which is also referred to as O—N layers. Asilicon nitride layer 2340 overlays the bottom dielectric structure2330, an oxide layer 2350 overlays the silicon nitride 2340, and ap-poly 2360 overlays the oxide layer 2350. Other suitable materials canbe implemented in place of the p-poly layer 2360, such as n-poly or ametal gate. A gate voltage 2370 Vg is applied to the p-poly 2360, and asubstrate voltage 2376 Vsub is applied to the p-type poly substrate2310. A drain voltage Vd 2372 is applied to the drain n+ doped region2320, and a source voltage Vs 2374 is applied to the source n+ dopedregion 2322.

FIG. 24 illustrates a fourth embodiment of a bottom oxide with amulti-layer dielectric structure implemented in a MONONOS TFT memory2400 on a poly substrate for use in a turn-on mode operation. TheMONONOS TFT memory 2400 is fabricated on a p-type poly substrate 2410with a drain n+ doped region 2420 and a source n+ doped region 2422 thatare formed on the upper right side and the upper left side of the p-typepoly substrate 2410, respectively. A bottom dielectric structure 2430overlays the p-type poly substrate 2410. The bottom dielectric structure2430 has multiple layers comprising an oxide 2436 overlaying a siliconnitride layer 2434, and the silicon nitride layer 2434 overlaying theoxide 2432, which is also referred to as O—N—O layers. A silicon nitridelayer 2440 overlays the bottom dielectric structure 2430, an oxide layer2450 overlays the silicon nitride 2440, and a p-poly 2460 overlays theoxide layer 2450. Other suitable materials can be implemented in placeof the p-poly layer 2460, such as n-poly or a metal gate. A gate voltage2470 Vg is applied to the p-poly 2460, and a substrate voltage 2476 Vsubis applied to the p-type poly substrate 2410. A drain voltage Vd 2472 isapplied to the drain n+ doped region 2420, and a source voltage Vs 2474is applied to the source n+ doped region 2422.

Turning now to FIG. 25, there is shown a first embodiment of an M(HK)NOSmemory 2500 having two bits per cell with a high-K material stack on asilicon substrate for use in a turn-on mode operation. The M(HK)NOSmemory 2500 is fabricated on a p-type silicon substrate 2510 with adrain n+ doped region 2520 and a source n+ doped region 2522 that areformed on the upper right side and the upper left side of the p-typesilicon substrate 2510, respectively. A bottom dielectric layer 2530comprising an oxide layer overlies the p-type silicon substrate 2510,and a charge trapping layer 2540 comprising a silicon nitride layeroverlies the bottom dielectric layer 2530. A high-K material stack 2550is disposed over the charge trapping layer 2540, and a p-poly layer 2560is disposed over the high-K material stack 2550. A gate voltage 2570 Vgis applied to the p-poly 2560, and a substrate voltage 2576 Vsub isapplied to the p-type silicon substrate 2510. A drain voltage Vd 2572 isapplied to the drain n+ doped region 2520, and a source voltage Vs 2574is applied to the source n+ doped region 2522.

The high-K material stack 2550 is selected from a dielectric materialthat possesses a higher dielectric constant than the bottom dielectriclayer 2530 in one embodiment. The bottom dielectric material 2530 may beimplemented with silicon dioxide, SiO₂, which has a dielectric constantk value of about 3.9. A high-K material increases capacitance, orremains unchanged in the reduced area of a MOS gate and a gatedielectric so that it is sufficiently thick to prevent excessivetunneling current. In another embodiment, the high-K material stack 2550is selected from a dielectric material that possesses a higherdielectric constant than the charge trapping layer 2540. Some examplesof suitable high-K dielectric materials 2550 include aluminum oxideAl₂O₃, and hafnium oxide HfO₂. The description of the high-K materialstack is also applicable to the embodiment described with respect toFIG. 26.

FIG. 26 illustrates a second embodiment of an M(HK)NOS memory structure2600 with a high-K material stack on a poly substrate for use in aturn-on mode operation. The M(HK)NOS memory 2600 is fabricated on ap-type poly substrate 2610 with a drain n+ doped region 2620 and asource n+ doped region 2622 that are formed on the upper right side andthe upper left side of the p-type silicon substrate 2610. A bottomdielectric layer 2630 overlies the p-type poly substrate 2610, and asilicon nitride layer 2640 overlies the bottom dielectric layer 2630. Ahigh-K material stack 2650 is disposed over the silicon nitride layer2640, and a p-poly layer 2660 is disposed over the high-K material stack2650. A gate voltage 2670, Vg, is applied to the p-poly 2660, and asubstrate voltage 2676, Vsub, is applied to the p-type poly substrate2610. A drain voltage Vd 2672 is applied to the drain n+ doped region2620, and a source voltage Vs 2674 is applied to the source n+ dopedregion 2622.

FIGS. 27A-27C are structural diagrams illustrating a first method forincreasing a second bit window of an M(HK)NOS memory 2500 or 2600 with ahigh-K material stack on either a silicon substrate or a poly substratefor use in a turn-on mode operation. FIG. 27A is a structural diagramillustrating the programming of the M(HK)NOS memory 2500 or 2600 bychannel hot electron at a right bit location. A directional arrow 2710indicates that the channel hot electron is applied to the right bit, asshown with electrons 2720 in the charge trapping layer 2540. The gatevoltage Vg 2570 is applied with 8 volts, the drain voltage Vd 2574 isapplied with 5 volts, the source voltage Vs 2576 is applied with 0volts, and the substrate voltage Vsub 2572 is applied with 0 volts. Thecombination of these applied voltages results in channel hot electron ofthe right bit in the M(HK)NOS memory 2500 or 2600 to a positive voltagethreshold +Vt.

FIG. 27B is a structural diagram illustrating the programming of theM(HK)NOS memory 2500 or 2600 by channel hot electron at a left bitlocation. A directional arrow 2730 indicates that the channel hotelectron is applied to the left bit, as shown with electrons 2740 in thecharge trapping layer 2540, The gate voltage Vg 2570 is applied with 8volts, the drain voltage Vd 2574 is applied with 0 volts, the sourcevoltage Vs 2576 is applied with 5 volts, and the substrate voltage Vsub2572 is applied with 0 volts. The combination of these applied voltagesresults in channel hot electron of the left bit in the M(HK)NOS memory2500 or 2600 to a positive voltage threshold +Vt. FIG. 27C is astructural diagram illustrating a hole injection erase of the M(HK)NOSmemory 2500 or 2600 by hole tunneling. During the erase operation, thehole tunneling erase is carried out on the left bit by moving holecharges 2760 a through the p-type substrate 2510 (either a p-typesilicon substrate or a p-type poly substrate), and through the bottomdielectric layer 2530 and into the charge trapping layer 2540. The holetunneling erase is also carried out on a right bit in a direction asindicate by an arrow 2750 by moving hole charges 2760 b through thep-type substrate 2510 (either a p-type silicon substrate or a p-typepoly substrate), the bottom dielectric layer 2530, and into the chargetrapping layer 2540. The gate voltage Vg 2570 is applied with a negativevoltage of −16 volts, the drain voltage Vd 2574 is applied with 0 volts,the source voltage Vs 2576 is applied with 0 volts, and the substratevoltage Vsub 2572 is applied with 0 volts. The combination of theseapplied voltages causes hole injection erase by hole tunneling by movinghole charges through the p-type substrate 2510, the bottom dielectriclayer 2530, and into the charge trapping layer 2540.

FIGS. 28A-28C are structural diagrams illustrating a second method forincreasing a second bit window of an M(HK)NOS memory 2500 or 2600 with ahigh-K material stack on either a silicon substrate or a poly substratefor use in a turn-on mode operation. FIG. 28A is a structural diagramillustrating the programming of the M(HK)NOS memory 2500 or 2600 bychannel hot electron at a right bit location. A directional arrow 2810indicates that the channel hot electron is applied to the right bit, asshown with electrons 2820 in the charge trapping layer 2540. The gatevoltage Vg 2570 is applied with 8 volts, the drain voltage Vd 2574 isapplied with 5 volts, the source voltage Vs 2576 is applied with 0volts, and the substrate voltage Vsub 2572 is applied with 0 volts. Thecombination of these applied voltages results in channel hot electron ofthe right bit in the M(HK)NOS memory 2500 or 2600 to a positive voltagethreshold +Vt.

FIG. 28B is a structural diagram illustrating the programming of theM(HK)NOS memory 2500 or 2600 by channel hot electron at a left bitlocation. A directional arrow 2830 indicates that the channel hotelectron is applied to the left bit, as shown with electrons 2840 in thecharge trapping layer 2540. The gate voltage Vg 2570 is applied with 8volts, the drain voltage Vd 2574 is applied with 0 volts, the sourcevoltage Vs 2576 is applied with 5 volts, and the substrate voltage Vsub2572 is applied with 0 volts. The combination of these applied voltagesresults in channel hot electron of the left bit in the M(HK)NOS memory2500 or 2600 to a positive voltage threshold +Vt.

FIG. 28C is a structural diagram illustrating a hole injection erase ofthe M(HK)NOS memory 2500 or 2600 by hole tunneling. During the eraseoperation, the hole tunneling erase is carried out on the left bit in adirection as indicate by an arrow 2850 by moving hole charges 2860 athrough a left bit by moving hole charges 2860 a though the p-poly 2560,the high-K material 2550, and into the charge trapping layer 2540. Thehole tunneling erase is also carried out on a right bit by moving holecharges 2860 b though the p-poly 2560, the high-K material 2550, andinto the charge trapping layer 2540. The gate voltage Vg 2570 is appliedwith a negative voltage of −8 volts, the drain voltage Vd 2574 isapplied with 8 volts, the source voltage Vs 2576 is applied with 8volts, and the substrate voltage Vsub 2572 is applied with 8 volts. Thecombination of these applied voltages causes hole injection erase byhole tunneling in moving hole charges through the p-type substrate 2510,the bottom dielectric layer 2530, and into the charge trapping layer2540.

FIG. 29A is a structural diagram illustrating the programming of theleft bit in the M(HK)NOS memory 2500 or the M(HK)NOS TFT memory 2600,and FIG. 29B is a corresponding graphical diagram of a two-bit-per-celloperation window that illustrates the second bit effect which pertainsto the right bit in this instance. A second bit effect occurs in amemory cell that employs a two-bit operation, i.e. a left bit and rightbit. When one of the two bits is programmed, the voltage threshold forthe other bit may also increase, even though only one bit is programmed.The programming of a left bit is illustrated in FIG. 29A with anindication of charges 2910 on a left bit 2912. Although only the leftbit 2912 is programmed, the programming of the left bit 2912 also causesthe voltage threshold of a right bit 2914 to increase, as shown in FIG.29B. A curve 2920 illustrates that the voltage threshold of right bit2914 increases as the left bit 2912 is programmed. Such phenomenon isreferred to as a second bit effect. An ideal curve, without the secondbit effect, would include continuing programming of a left bit whichwould cause the voltage threshold of the left bit to increase but thevoltage threshold of the right bit would not be affected such that thevoltage threshold of the right bit would remain substantially constant.

In addition to the erase operations described above with respect tovarious embodiments, the present invention can also be applied as apre-program erase step as described in the following flow diagrams. FIG.30 is a flow diagram illustrating the process 3000 to pre-program eraseSONOS-type or TFT-SONOS memories. At step 3010, a memory structurecomprising a SONOS-type or TFT-SONOS memory having two-bits-per-cell ispre-program erased to a negative voltage threshold, −Vt, by applying apositive gate voltage, +Vg, using hole tunneling erase from a gateterminal of a SONOS-type or TFT-SONOS memory. At step 3020, theSONOS-type or TFT-SONOS memory is programmed by channel hot electron toa left bit and a right bit of the charge trapping memory. At step 3030,the SONOS-type or TFT-SONOS memory is erased either by a hole injectiontechnique or a band-to-band hot hole technique. Alternatively at step3010, in some embodiments, the pre-program erase is implemented using aband-to-band hot hole erase instead of the hole tunneling technique. Inother embodiments at step 3010, the hole tunneling technique in thepre-program erase operation erases the SONOS-type or TFT-SONOS memory toa voltage level that is lower than an initial voltage threshold, Vt(i).

FIG. 31 is a flow diagram illustrating the process 3100 of pre-programerasing of SONOS-type or TFT-SONOS memories. At step 3110, a memorystructure comprising a SONOS-type or TFT-SONOS memory havingtwo-bits-per-cell is pre-program erased to a negative voltage threshold,−Vt, by applying a positive gate voltage, −Vg, using hole tunnelingerase from a gate terminal of a SONOS-type or TFT-SONOS memory. At step3120, the SONOS-type or TFT-SONOS memory is programmed by channel hotelectron to a left bit and a right bit of the memory cell. At step 3130,the SONOS-type or TFT-SONOS memory is erased either by a hole injectiontechnique or a band-to-band hot hole technique. Alternatively at step3110 in some embodiments, the pre-program erase is implemented using aband-to-band hot hole erase instead of the hole tunneling technique. Inother embodiments at step 3110, the hole tunneling technique in thepre-program erase erases the SONOS-type or TFT-SONOS memory to a voltagelevel that is lower than an initial voltage threshold, Vt(i).

FIG. 32 is a flow diagram illustrating the process 3200 of pre-programerasing a SONOS-type or TFT-SONOS memory comprising a top gate oxidehaving a multi-layer stack where each memory cell has two bits per cell.At step 3210, the SONOS-type or TFT-SONOS memory structure with themulti-layer stack is erased to a negative voltage threshold, −Vt, byapplying a positive gate voltage, +Vg, using hole tunneling erase from agate terminal of a SONOS-type or TFT-SONOS memory. At step 3220, theSONOS-type or TFT-SONOS memory is programmed by channel hot electron toa left bit and a right bit of the memory cell. At step 3230, theSONOS-type or TFT-SONOS memory is erased either by a hole injectiontechnique or a band-to-band hot hole technique. Alternatively at step3210 in some embodiments, the pre-program erase is implemented using aband-to-band hot hole erase instead of the hole tunneling technique. Inother embodiments at step 3210, the hole tunneling technique in thepre-program erase erases the SONOS-type or TFT-SONOS memory to a voltagelevel that is lower than an initial voltage threshold, Vt(i). In afurther embodiment at step 3210, the SONOS-type or TFT-SONOS memorystructure with the multi-layer stack is erased to a negative voltagethreshold, −Vt, by applying a negative gate voltage, −Vg, using holetunneling erase from substrate of the SONOS-type or TFT-SONOS memory.

FIG. 33 is a flow diagram illustrating the process 3300 of pre-programerasing a SONOS-type or TFT-SONOS memory comprising a bottom gate oxidehaving a multi-layer stack where each memory cell has two bits per cell.At step 3310, the SONOS-type or TFT-SONOS memory structure with themulti-layer stack is erased to a negative voltage threshold, −Vt, byapplying a positive gate voltage, +Vg, using hole tunneling erase from agate terminal of a SONOS-type or TFT-SONOS memory. At step 3320, theSONOS-type or TFT-SONOS memory is programmed by channel hot electron toa left bit and a right bit of the memory cell. At step 3330, theSONOS-type or TFT-SONOS memory is erased either by a hole injectiontechnique or a band-to-band hot hole technique. Alternatively at step3310 in some embodiments, the pre-program erase is implemented using aband-to-band hot hole erase instead of the hole tunneling technique. Inother embodiments at step 3310, the hole tunneling technique in thepre-program erase erases the SONOS-type or TFT-SONOS memory to a voltagelevel that is lower than an initial voltage threshold, Vt(i). In afurther embodiment at step 3310, the SONOS-type or TFT-SONOS memorystructure with the multi-layer stack is erased to a negative voltagethreshold, −Vt, by applying a negative gate voltage, −Vg, using holetunneling erase from the substrate of the SONOS-type or TFT-SONOSmemory.

FIG. 34 is a flow diagram illustrates the process 3400 of pre-programerasing a SONOS-type or TFT-SONOS memory comprising a high-K materialwhere each memory cell has two bits per cell. At step 3410, theSONOS-type or TFT-SONOS memory structure with the high-k material iserased to a negative voltage threshold, −Vt, by applying a positive gatevoltage, +Vg, using hole tunneling erase from a gate terminal of aSONOS-type or TFT-SONOS memory. At step 3420, the SONOS-type orTFT-SONOS memory is programmed by channel hot electron to a left bit anda right bit of the memory cell. At step 3430, the SONOS-type orTFT-SONOS memory is erased either by a hole injection technique or aband-to-band hot hole technique. Alternatively at step 3410 in someembodiments, the pre-program erase is implemented using a band-to-bandhot hole erase instead of the hole tunneling technique. In otherembodiments at step 3410, the hole tunneling technique in thepre-program erase erases the SONOS-type or TFT-SONOS memory to a voltagelevel that is lower than an initial voltage threshold, Vt(i). In afurther embodiment at step 3410, the SONOS-type or TFT-SONOS memorystructure with the multi-layer stack is erased to a negative voltagethreshold, −Vt, by applying a negative gate voltage, −Vg, using holetunneling erase from substrate of the SONOS-type or TFT-SONOS memory.

The invention has been described with reference to specific exemplaryembodiments. For example, the method in the present invention isapplicable to any type or variation of a nitride trapping memoryincluding both N-channel and P-channel SONOS types of devices andfloating gate memory. Various modifications, adaptations, and changesmay be made without departing from the spirit and scope of theinvention. Accordingly, the specification and drawings are to beregarded as illustrative of the principles of this invention rather thanrestrictive, the invention is defined by the following appended claims.

1. A method for programming a multi-bit memory cell having a chargetrapping structure disposed between a gate and a substrate, and having aright bit charge storage site and a left bit charge storage site, themethod comprising: setting both of the right bit charge storage site andthe left bit charge storage site of the multi-bit memory cell to anegative threshold voltage; and after said setting, programming said oneof the right bit and the left bit to a positive threshold voltage by aprogramming operation, said programming operation maintaining another ofthe right bit and the left bit to a negative threshold voltage.
 2. Themethod of claim 1, wherein the setting comprises hole tunneling from thegate.
 3. The method of claim 1, wherein the setting comprises holetunneling from the substrate.
 4. The method of claim 1, wherein thesetting comprises a band-to-band hot hole tunneling.
 5. The method ofclaim 1, wherein the programming operation comprises channel hotelectron programming said one of the right bit and the left bit.
 6. Themethod of claim 1, wherein after said programming operation a differencein threshold voltages between the first and second bit charge storagesites is greater than or equal to 4 Volts.
 7. The method of claim 6,wherein after said programming operation the first bit charge storagesite has a threshold voltage greater than or equal to +3 Volts, and thesecond bit charge storage site has a threshold voltage less than orequal to −1 Volt.